I would imagine that fat tissue has less blood passing through it, whereas muscle tissue is full of warming blood vessels that feed it oxygen and take away potentially poisonous metabolites. Also, the metabolic process in itself generates heat. Compared to muscle, fat is relatively inert metabolically.Perhaps, though in truth I have no idea, fat has some innate thermodynamic quality that allows it to dissipate heat, perhaps necessary to offset any undesirable over-insulation.

One reason is due to density. Fat is of lesser density than muscle. In a given volume of fat there are less cells to hold heat. The individual cells themselves are also of lesser density. By contrast muscle for the same volume of tissue has more cell units packed into the same volume therefore more cell to hold more units of heat.The increased density also slows thermal radiation ie. heat escape. Another factor responsible for heat differences between muscle and fat is the greater metabolic activity of muscle. Activity implies energy exchange therefore muscle has more heat to give off. The lower density of fats means that it also loses whatever heat is has at a faster rate.

The warmth/heat felt is due to the dissipation of heat from the cells/tissues, whereas fat being insulator and meant for insulating the body from external cold and to prevent excessive loss of heat energy, doesnt let the body heat to be transmitted outside, so is relatively colder than body parts with lesser fat.

Blubber is an excellent heat insulator. This is why whales and other marine mammals have a thick layer of blubber under their skin to insulate their warm blood from the cold ocean. Similarly where your body is now more insulated from the outside air, it will feel colder as there is less heat leaking out.

The following answer has been selected and edited by New Scientist staff

To put it simply, fat is an insulator. In the same way that walls insulate a home, fat decreases the rate of thermal loss so its outside edge feels cooler. If you touch a person wearing a winter jacket, you will find it cooler than their skin.

The following answer has been selected and edited by New Scientist staff

When ambient temperature is lower than that of the body, the temperature of the skin depends on the balance between the amount of heat reaching the skin, either via warm arterial blood or by direct conduction from underlying tissues, and the amount of heat leaving the skin to the environment.

When we get cold, blood vessels under the skin close up, reducing the delivery of heat to the skin by arterial blood. The skin is then kept warm by heat conduction alone. Fat, or adipose tissue, is a better insulator than lean tissue because it contains less water. Thus any skin above adipose tissue will receive less heat, making it cooler than the skin over lean tissue.

Many animals, especially marine animals, exploit this property of adipose tissue by having a thick layer of subcutaneous fat. When these animals need to conserve heat, the blood vessels below the surface close, as in people, and the thick insulating layer of fat minimises heat conduction to the skin. When heat dissipation is needed, during exercise for example, the flow of arterial blood to the skin is increased, effectively bypassing the subcutaneous insulation.

Shane Maloney, School of Biomedical and Chemical Science, University of Western Australia, Perth

I think this may be due to the fact that water has a higher specific heat capacity than fat and as fat cells would most likely contain more fat per unit of mass and possibly less water than musccle tissue.

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